Water Vapor Isotopic Signatures of Moisture Transport in the UT/LS

Convection carries substantial moisture, in the form of vapor and ice, into the UT/LS. However, predicting the contribution to atmospheric water vapor is difficult: mixing and sedimentation determine the quantity and location of ice sublimation. Measurements of the isotopic composition of water vapor to heavy water vapor diagnoses sublimation and the processes leading up to it. These processes must be understood in order to make accurate predictions of cloud microphysics in a changing climate. Integrated cavity output spectroscopy exploits longer path length than previously possible, allowing for measurements in regions of low mixing ratios for both water vapor and its isotopologues. Three recent aircraft campaigns provide the opportunity for detailed case studies. All campaigns flew out of Houston during the summer and took place during the North American Monsoon: AVE-WIIF (2005) and ACCLIP test and science flights (2021 and 2022). We find examples of moisture transport via mixing and sedimentation. In AVE-WIIF, fresh, isotopically enriched ice sediments rapidly into sub-stratospheric subsaturated air, leading to an isotopic depletion at the level of detrainment, and an isotopic enrichment below the level of detrainment. In ACCLIP, an aged convective plume shows an enriched isotopic vapor signal, suggesting that there was sufficient time for the ice to sublimate and mix with subsaturated air below the tropopause. These results suggest that multiple processes add moisture to the UT/LS, and that isotopic measurements help identify these processes.